Application of detached-eddy simulation to free surface flow over dunes

Abstract

Detached-eddy simulation (DES) is proposed for simulating shallow water flow in the present paper. Compared to the traditional numerical model used in river flow simulation that is based on hydrostatic assumption, the non-hydrostatic pressure terms are introduced into the momentum equations to improve the accuracy of simulating flow over a distinct, uneven bottom. The numerical scheme is a finite volume method based on an unstructured grid in the horizontal plane, and the σ coordinate in the vertical direction to fix the free surface and the uneven bottom. The in-house codes are paralleled by OpenMP. While most of the domain (including the near bottom zone and the upstream and downstream boundary zones) is designed as a Reynolds-Averaged Navier Stokes (RANS) zone, only a local computational zone is simulated by large-eddy simulation (LES), which is implemented by means of properly designing the grid scales. A case study of flow over a series of five dunes was used to validate the model, focusing on the influence of the inflow condition on the small-scale vortical structures. As an improved method to inspire much more sufficient velocity fluctuation, a zonal detached-eddy simulation (ZDES) technique was introduced into the present model. The same case study was also carried out in a RANS model for comparison with the hybrid RANS and DES or ZDES results. The proposed model is shown to be equally effective in the prediction of small-scale vortical structures in shallow water flow with free surface, and to have potential for simulating large-scale flows, such as natural river flows.